New HRSTEM approach overcomes a long-standing geometrical limitation in atomic-scale defect analysis.
Stacking faults are among the most important planar defects in crystalline materials. They can strongly influence mechanical, electronic and optical properties and play a particularly important role in materials such as superalloys and semiconductors. Understanding their atomic structure is therefore essential—but until now, high-resolution electron microscopy has faced a fundamental geometrical limitation: stacking faults could only be analyzed reliably when viewed edge-on.
Researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), together with collaborators from The Ohio State University, have now developed a high-resolution scanning transmission electron microscopy (HRSTEM) approach that allows stacking faults to be analyzed even when they are inclined with respect to the electron beam. The work has been published in Nature Communications.
The new method exploits a characteristic contrast that appears where an inclined stacking fault intersects the surface of a thin TEM specimen. In this region, the crystal lattices above and below the fault overlap in projection. Their relative displacement provides direct information about the atomic structure of the defect and makes it possible to distinguish different types of stacking faults.
This removes an important restriction of conventional atomic-resolution analysis. In face-centered cubic materials, for example, only part of the relevant stacking-fault planes can normally be brought into an edge-on orientation for a given crystallographic viewing direction. Combining edge-on and inclined-fault analysis now provides access to all relevant glide planes along commonly used zone axes.
The researchers demonstrated the approach in a CoNi-based superalloy, where stacking faults play a key role in deformation. The method remains applicable even for overlapping defects and specimen thicknesses above 100 nanometers. It can also be extended to the partial dislocations bounding the faults, providing insight into how the defects formed. Multislice and probe-propagation simulations further revealed that fault-induced electron de-channeling is responsible for the characteristic image contrast.
Importantly, the concept is not limited to superalloys. The study demonstrates its applicability to different materials and crystal structures, including an oxide-dispersion-strengthened alloy and a semiconductor with sphalerite structure, making the approach broadly relevant for atomic-scale defect analysis in crystalline materials.
The work was carried out by Nicolas Karpstein and colleagues at the Institute of Micro- and Nanostructure Research (IMN), in collaboration with researchers from the Chair of General Materials Properties at FAU and The Ohio State University.